a/c man

Martin if you could tell me a good cam size for my car street car that runs at the track and I dont like too much lope its a 402 with 9.3 compression with afr230s, YSI 16# boost, ls6 intake, headers catless, auto 2800stall right now its cam is off the shelf comp makes good power just think It could be better.

228/230 581/592 on a 114 lsa XER.

Thank you scotty

PEETYZ

Re dynoed buddies car after plug was fixed! She made 577/508! Not bad for an NA cam

a/c man

bmfrc

The easiest correlation of blower/spray cam,vs n/a:

Begin with a blown 8cyl.OHV(16) single cam. This engine is going to need a mild cam with lobe centers, higher than n/a. Also,this translates to the amount of time the referenced valves are open simultaneously(at the same time). Lobe lift choices are forgiving as driver/ builder
preferences vary. The higher the LSA#, on the grind, the tamer the idle.Lobelift is very important in relation to cubic inch displacement. Again, the larger the CID,YOU CAN go up on lift/ duration,yet LSA needs to stay as we discussed.
The simplest way to remember a vast number of choices is to keep in
mind that AN AGRESSIVE PROFILE IS NOT REQUIRED FOR BOOST OR
SPRAY. RULE OF THUMB IS POWER ADDERS WILL REPLACE WHAT ISNT THERE PROVIDED THE LSA IS AS STATED.
The worst possible scenariois when a buddies cam is used because it was a great buy,or "it ran great in my car man" run it ........hope this gives some insight, on a topic often debated, due to lack of understanding.
Lamar Murphy
40 yr Veteran Chevrolet Racing Engine Developement

0Matt@Tick

Scotty,

I would be most glad to recommend a custom grind for you. With what you have stated, I feel you would like this camshaft best:

231/247 .631/.632 117+4

The lobes have great high lift cylinder fill area, but their qualities in terms of acceleration, velocity and jerk are much lower than other high lift lobes. I have been using these lobes for over a year after Billy Godbold@Comp introduced them to me. The idle quality would be smooth for a 403, and this profile would make considerably more power than your current cam. Your current cam has 1 degree valve overlap@.050". Mine has 5.

With talk of "slow" lobes being better recently on another site, it's ironic I've been using these "slow" lobes for over a year. I always use a milder exhaust lobe than intake lobe as the exhaust port cares little about area(lift) and more about timing events(exhaust center line/duration). Even on N/A cam profiles I do the same.

A centrifugal blower motor produces higher VE at higher piston speeds(RPM) than a similar sized N/A engine. If you've ever seen a dyno graph for a centrifugal blower motor, the torque curve always peaks higher than a similar sized N/A engine.

Higher VE = higher cylinder fill. Higher cylinder fill = higher exhaust volume. Higher RPM means the exhaust port has a shorter amount of time to evacuate the cylinder. Thus needing more time to blow down/evacuate. This necessitates an earlier opening exhaust valve event.

Valve events are directly related to piston speed. Piston speed is crucial in determining a cam profile.

Overlap and where we close the exhaust valve and open the intake valve is directly related to piston speed. How much time do the two ports need to "talk" to one another for a given amount of VE to sustain at a given piston speed?

In a blower motor, we have pressure higher than atmospheric in the intake port, and atmospheric pressure in the exhaust. Anyone who sat through a high school physics class can tell you that high pressure will always move to low pressure. So in a blower motor during overlap when both the valves are open at the same time, because of this pressure differential that exists, scavenging is increased tremendously. It doesn't need near as much time(duration) to give that "supercharging effect" overlap is known for. Overlap has been called a "5th stroke" due to its supercharging effect. When you have a supercharger though, you don't need it as badly.

I don't like to use a lot of overlap on lower RPM street motors. I just don't feel it's needed. I've tried higher overlap and lower overlap and the lower overlap stuff makes more power every single time. This is in a street engine and not something that turns 8000rpm+.

This is just my philosophy on why I cam a centrifugal the way I do. Big split, wide LSA can sum up pretty much how I do it.

I agree with a lot of what you said. Mainly lobe profiles and what is needed with boost. I've found high lift profiles with mild ramps really help to fill the cylinder that much more. Added duration isn't needed to fill the cylinder as it creates unnecessary valve overlap in a forced induction engine. Lift is highly beneficial though.

Adding lift doesn't mean the lobe gets more aggressive . There are many ways to keep the lobe mild while adding lift.

Keep in mind though one thing about LSA. It isn't something we need to concentrate on other than to identify a certain cam profile. It's a mathematical equation of two known center lines. We add our intake center line(which is where max intake lobe lift occurs after top dead center) to our exhaust center line (which is where max exhaust lobe lift occurs before top dead center) and we divide the sum of those two numbers by 2. This gives us our lobe separation in cam degrees as the center lines are expressed in crank degrees. Thus the need to divide by two.

For my recommendation above: 117+4= 113 ICL + 121 ECL = 234/2=117LSA

I don't think in terms of LSA. I don't use it to grind a cam profile. I choose valve events and those events dictate certain intake and exhaust durations on given intake and exhaust center lines.

I hope some of what I've said helps to answer a lot of the questions in here. It's pretty late and I'm ready to go to bed now though lol.

Milan

^ good post....very informative

a/c man

Martin@Tick Wow your really shed some light on cams, I like the cam you chose for my setup what lobes do you like? here is a cam I have now that I was getting ready to try next time I'm in the motor, how would it perform compared to my current cam? and the one you suggested, Be honest I can take it :-)

226/242 615/600 on a 117LSA Is it worth a cam swap?

Thank You, Martin@Tick for sharing all this knowledge.

_zebra

that's some great info - i've been trying to learn more about this type stuff over the years. how would what you recommended above change with a PD blower?

also, i'm trying to max-perform my current TVS2300 on my LS1 when i get a chance to build it, but i also want some lope to the idle... which i know i'll probably have to sacrifice some power to get the sound, right?

0Matt@Tick

Do you know the intake center line for that camshaft? Since the exhaust center line in a single cam motor is fixed to the intake center line, I only need to know the intake center to determine it's timing events.

It actually looks like a smaller version of my camshaft I specified for you.

Without seeing the timing events though I couldn't compare them accurately.
A PD blower does not sustain VE very well at high engine speeds unless it is very large. The blower cannot sustain the air flow demand of the engine and thus boost and power drops off as RPM rises. Thus adding a larger camshaft, with added valve overlap is not beneficial. This will create even more airflow demand at higher engine speeds draining the blower that much sooner.

The only way I have found to counter act this is to open the exhaust valve sooner to extend the torque curve. Opening the exhaust valve too soon by some can be seen as blowing torque out the tail pipe.

In a PD blower application, have you ever seen or heard someway say, "Man, I just don't have enough torque with this PD blower." "I wish I had some more torque."

You don't! Thus I don't worry about "blowing torque out the tail pipe" in such a combination.

Here is a good thread where I've also outlined some blower cam FAQ in two of my posts.
http://ls1tech.com/forums/17755759-post17.html

http://ls1tech.com/forums/17755759-post17.html

Slow Z06

just out of curiousity, for the turbo guys

347ci TT Z06 6spd car
9.5:1 cr running up to 30psi

Needs to have some street ability as that is where it lives 98% of its life, but want some lope to it. its a weekend car now. no longer daily driven. it makes insane numbers as it is and Im running a stock LS7 cam. What would you recommend?

ajrothm

Great posts Martin! Good info.....

0Matt@Tick

What heads? What intake? What size compressor and turbine wheel and AR exhaust housing are you running as well?

a/c man

Martin thats the only #s I have for that cam except for the grind # on the cam from BULLET thanks for helping

_zebra

okay, thanks. is that where the recommendation for 5° or more intake/exhaust split comes from?

shoot... with 30psi, it almost seems like you could afford to lope it a bit & not lose too much

Slow Z06

Stock heads. Ls6 intake. 62/65 dbb. .84ar

Slow Z06

Not too much worried about loosing any power. Just don't want to freak everything out due to improper valve events. I forgot to mention its a e85 car with lots if meth added.

0Matt@Tick

Without knowing the center lines it's hard to compare, but judging by the duration and LSA, if you already have that cam in your possession I'd try it. It seems to be a smaller version of what I recommended.
I do not spec cams based on "duration split" or "it has to have this LSA". I choose the valve events I want to see and then those compute to given center lines and durations which determines LSA and the cam profile.

The I/E % and how they compare to one another determines "split" and center lines. Not, "it has to have this much no ifs ands or buts".

Let's take your combo for example.

I'd want a 43-46 IVC depending on certain parameters. The IVC event in my world dictates the EVO event, the EVC event and the IVO event. The IVC event is directly related to where the engine will peak, so it determines where the exhaust valve needs to open. This IVC event that is determining for the most part(the other events are important) where the engine peaks will also determine how long the two ports need to communicate with each other during overlap to sustain that VE at that RPM. With a supercharger as I mentioned it's not needed as much.

With that 43-46 IVC event and a cathedral port cylinder head I'd want to see a 59-64 EVO event. If the IVC was 43, I'd want a 59 EVO. If the IVC was 46 I'd want a 64 EVO. These are @.050" FWIW.

With that said, and the size of your blower and your request for some lope at idle I would specify -2 to 0 degrees of overlap be used. To help the blower on the top end, I would close the exhaust valve@TDC to help scavenge spent exhaust gas at high RPM.

Here is what our two proposed camshafts would look like then @.050:

59 EVO
-0.5 EVC
-0.5 IVO
43 IVC

64 EVO
-2 EVC
2 IVO
46 IVC

This is how I spec a cam. Once I've reached this step, I convert timing events @.050 to duration@.050 and ICL/ECL.

The 59evo, 0evc, 0ivo, 43ivc timing events would give us a 223/239 116+4 profile.

The 64evo, -2evc, 2ivo, 46ivc timing events would give us a 228/244 118+6.

Hope this helps.

0Matt@Tick

With a more restrictive turbine I open the exhaust valve earlier to move more exhaust gas before bottom dead center.

Since the smaller turbine has higher exhaust pressure it cannot scavenge spent exhaust gasses efficiently. Because of this the exhaust valve needs to close much earlier than in a N/A motor. Because of this, we cannot evacuate as much volume in terms of spent exhaust gas from the earlier closing exhaust event if we do not compensate by opening the exhaust valve earlier.

Some like reverse grind cams for high back pressure. This opens the exhaust valve later and closes it earlier in a general sense. IMO this does not move the exhaust gas needed to sustain power at higher piston speeds.

I like to use a more traditional split with high back pressure camshafts with wide exhaust center lines to open the exhaust valve sooner(move more exhaust gas BBDC) and close it earlier BTDC to keep reversion from occurring. Also since the engine cannot scavenge effectively at higher RPM, adding overlap which benefits from scavenging is not beneficial, it is detrimental. What happens is the small turbine/turbo cannot supply the air flow the engine is demanding at high RPM because the compressor is demanding so much work from the turbine. The compressor wheel tells the turbine work needs to be done, but the turbine is so small it can't do any more work. This pushes the compressor way over speed and off the compressor map. This is where power plummets. This can easily happen with a stock camshaft, so it's a fine line when grinding a cam for a small turbine combination as you want to add some overlap and add some duration and lift. Just not too much.

Putting a much larger cam in, with more overlap and more duration will only create more demand for air flow at that RPM making the turbo fall off even sooner! This is why a smaller cam 9/10 is best for a turbo engine. That is unless you have a large turbo that can supply the air flow demands of the engine at those engine speeds.

In this case you mentioned a 96mm turbine wheel or twin 68mm turbine wheels. In that scenario the turbo can supply the air flow demand of the engine. A larger cam can be used with more overlap because of this. It's still a fine line though!

With a larger turbine or two smaller turbines I normally use an earlier EVO event, but only because the cam is normally larger with a later IVC event. Remember how I mentioned in the post above, my IVC event will dictate where my EVO event, EVC event and IVO event occurs. Since the larger turbine wheel can effectively use more overlap I normally add intake and exhaust duration. Adding intake duration means that the exhaust opening event must move earlier to match it. In this case, the EVO would likely be earlier than a smaller turbine, but only due to adding intake duration and a resulting later IVC/earlier IVO event.

I use a later EVC event and an earlier IVO event which is where the added overlap comes from. Since the turbine can actually do the work the compressor is asking of it, the compressor isn't pushed off the map and can supply air flow keeping power climbing or at least sustained at higher engine speeds. This is where the larger cam is actually beneficial with the larger turbine.

Same can be said with twin turbos.

Slow Z06

What can would you recommend with a twin setup like mine vs the ls7 cam that is being used?

0Matt@Tick

227/231 .624/.598 115+5

55.5
-4.5
3.5
43.5

Since you're pushing 30psi you're demanding a lot of air flow from your compressor wheels. Because of this, if we use a cam with a lot of overlap or a large amount of duration the airflow demand at higher engine speeds will only increase.

This will push the compressor further from its higher efficiency islands where it is much more efficient in making power.

The cam I've recommended could be made larger if you were utilizing less of your compressor's airflow capabilities.

Hope this helps.